I have several data points that are plotted below and I would like to find the frequency value when the amplitude value crosses 4. I've included an example along with the data points in the example below. I've circled the answer graphically but I'm not sure how to compute it mathematically and get all the values for the frequencies I desire. How can I do this with octave / matlab? Also is there a mathematical term for what I'm trying to do?
In this example I'm trying to get 5 frequencies (but this is just an example) I know two answers are 30 and 80 but not sure how to get the rest. The full list could be thousands. I'm using octave 3.8.1
clear all,clf, clc,tic
%graphics_toolkit gnuplot %use this for now it's older but allows zoom
freq=[20,30,40,50,60,70,80];
amp_orig=[2,4,3,7,1,8,4];
amp_inv=[6,4,5,1,7,0,4];
plot(freq,amp_orig,'-bo')
hold on
plot(freq,amp_inv,'-r*')
xlabel ("Frequency");
ylabel ("Amplitude");
Thanks
Try this function from the File Exchange, it seems to work just fine in Octave. x0
are the frequencies of interest:
>> [x0,y0,iout,jout] = intersections(freq,amp_orig,freq,amp_inv)
x0 =
30.000
30.000
42.500
55.000
64.286
80.000
y0 =
4.0000
4.0000
4.0000
4.0000
4.0000
4.0000
iout =
2.0000
2.0000
3.2500
4.5000
5.4286
7.0000
jout =
2.0000
2.0000
3.2500
4.5000
5.4286
7.0000
Should you have access to Matlabs Mapping Toolbox, your problem can be solved easily using the function polyxpoly()
. It will find the intersections of two graphs. They do not even have to be lines, also polygons work. Here is an example for your case:
%graphics_toolkit gnuplot %use this for now it's older but allows zoom
freq=[20,30,40,50,60,70,80];
amp_orig=[2,4,3,7,1,8,4];
amp_inv=[6,4,5,1,7,0,4];
plot(freq,amp_orig,'-bo')
hold on
plot(freq,amp_inv,'-r*')
xlabel ('Frequency');
ylabel ('Amplitude');
%// find and add intersections
[xi,yi] = polyxpoly(freq,amp_orig,freq,amp_inv)
plot(xi, yi,'ro','markersize',20) %// draw the red circles automatically :-)
The result looks like this
来源:https://stackoverflow.com/questions/25984168/finding-where-plots-may-cross-with-octave-matlab